There are a number of applications where effective coupling of light into a light guide is desired. Some examples include different kinds of backlights and virtual displays designed to re-distribute light originally emitted e.g. by a LED (Light Emitting Diode).
A traditional approach is to feed the light into a plate-like light guide through its edge which gives a high coupling efficiency especially in the case of a light guide thicker than the beam size. However, in many applications it is the case that the thickness of the light guide is smaller than the size of the light beam to be coupled in. In these cases it is desirable to couple the light through the top or bottom surface of the light guide. The simplest solution is to use a diffraction grating on the side of incoming light of the light guide with a grating period suitable for in-coupling. This method is applicable as long as the lateral dimensions of the grating are up to about twice as large as the light guide thickness. In the case of reduced light guide thickness, the in-coupled light hits the grating area again after reflection at the opposite side of the light guide. Then, due to the reverse propagation of light, it is again coupled out, at the worst with the same efficiency as it was originally coupled in. This leads to great losses resulting in a poor total in-coupling efficiency. Thus, new optical designs must be used for achieving a sufficiently efficient coupling.
As one approach intended to avoid the problems described above, it is known to use radial grating geometry for a non-collimated LED light. With this kind of grating it has been possible to reduce the light guide thickness to 0.6 mm and this solution has also been successfully expanded to white light applications. However, in this kind of arrangement, the in-coupled light beam disperses in all directions in the plane of the waveguide without any possibility to confine its propagation in some particular direction.
Levola discloses in patent application US2005/0002611 A1 a structure for coupling light into a wave guiding substrate, the structure including a polarization converting element for changing the polarization state of the incoming light after the first interaction with the in-coupling grating. According to the description, changing the polarization from TE to TM between the first and the second interactions with the in-coupling grating makes it possible to design said grating so as to decrease reverse diffractions of the in-coupled light out from the light guide. As a result, the overall coupling efficiency is improved. It is reported that the coupling area can be made about two times wider than without the polarization element. However, an essential drawback of this solution is the limitation to specific polarization states of the light.
In solutions disclosed in patent publications JP11174270 and JP11281833, the coupling grating on a first surface of a waveguide formed on a substrate divides the incident light beam into two sub-beams propagating within the light guide with different directions. The basic idea of the disclosed solutions is that one of said sub-beams follows such a path that, after having been reflected at the second surface of the waveguide or from a buffer layer between the waveguide and the substrate, it is phase matched with another incident light beam and is coupled with it via an interferences phenomenon.
DE 4131738 A1 discloses an arrangement for in-coupling or out-coupling light between a waveguide and the ambient, the arrangement comprising two gratings. The waveguide has been formed on a glass plate substrate substantially thicker than the waveguide itself. There is a first grating on the surface of the waveguide guide serving as a redirecting element making light propagating within the waveguide diffract to the substrate. The light beam redirected or diffracted by the first grating has a predetermined intensity distribution. On the opposite surface of the substrate there is another grating serving as a phase correcting element correcting the phases of the light rays of the beam escaping the structure. The disclosed arrangement works also reversely for in-coupling. However, due to the thick substrate necessitated and the actually rather complex grating design needed to execute the functions described above, this particular approach is far from a generally applicable solution for coupling light into thin light guides.